In particular, choosing an improper initial condition or setting up a problem without a solution will simply cause the nonlinear solver to continue iterating without converging. How do/should administrators estimate the cost of producing an online introductory mathematics class? However, it is usually not possible to know this ahead of time. Solve the stationary study then the time dependent study. Click the link in the email we sent to to verify your email address and activate your job alert. A nonlinearity can be introduced into the model either in the governing equation, or by making any of the material properties, loads, or boundary conditions dependent upon the solution. It may also reveal that the model itself is ill-posed in some way. This case is generally difficult, or impossible, to solve since this material property is non-smooth. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. They are usually called comp1.u, comp1.v, and comp1.w though. The default Initial Values for the unknowns in most physics interfaces are zero. Is there a way to use the stationary solution obtained in Comsol 4.2 as the initial conditions in a time dependent model? Changes to these low-level settings from the defaults will usually be quite model- and case-specific. Within either of these features, it can also be helpful to enable the Results While Solving option, as shown in the screenshot below, to visualize the iterations being taken during the solution. Cooling and Solidification of Metal. Segregated approach and Direct vs. Iterative linear solvers, Time dependent function and stationary study, Combining Adaptive Mesh Refinement with Data Filtering, What to do when a linear stationary model is not solving, Galleria dei Modelli e delle App di Simulazione, 2023 da COMSOL. Use this parameter to modify the nonlinearity expressions in the model. Understanding the Fully Coupled vs. Load ramping and nonlinearity ramping can be used in combination, but start with only one or a few of the loads or nonlinearities being ramped. If it does so, use a finer increment in that range. What is the purpose of this D-shaped ring at the base of the tongue on my hiking boots? Extending this logic, if one wants to solve for any arbitrary load on a nonlinear system, it makes sense to solve a sequence of intermediate problems with gradually increasing load values and using the solutions from each previous step as the initial condition for the next step. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. This segregated approach is used by default for most 3D multiphysics models, and the software will automatically segregate the problem into appropriate groups. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. Stationary Engineer Jobs in Brea, California, United States, $87,400.00 What are people saying about cards & stationery in Brea, CA? SGP handled 7 different prints for me at once and they all came out perfectly, in a timely manner. I am following the same model as Comsol provide us on the web. What version of COMSOL are you using? The coupling terms between the different groups are thus neglected. Here we introduce the two classes of algorithms used to solve multiphysics finite element problems in COMSOL Multiphysics. Direct Iterative , Direct . The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. How to solve "Feature: Stationary Solver 1 (sol1/s1) Attempt - COMSOL - Not assigning proper boundary conditions: Especially if you have ports. Check the solver log to see if the continuation method is backtracking. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. Improving Convergence of Nonlinear Stationary Models - COMSOL This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. ", "Absolutely the best place to shop for flowers and greenery, I spent an hour just looking around for fun prior to purchasing a bouquet, they have the best customer service people all over the place and it's a huge place", "Recently came in here to pick up 45 chocolate Seegars (cigars) for a dinner banquet in honor of my baby nephew. Not meshing all the domains. COMSOL does not assume any legal liability for the accuracy of the data disclosed. Cecilia M. came from the backroom to help me and went out of her way to help me even though I had not called before going into the store. If you are unsure if your problem is linear and stationary, look at the Log. Numerically ill-conditioned means that the system matrix is nearly singular and that it will be difficult to solve on a finite-precision computer. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. She opened prepackaged 6-packs of Seegars to make ", All "stationary stores" results in Brea, CA, Search stationary stores in popular locations. That is, start by first solving a model with a small, but non-zero, load. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. - Posted Sep 9, 2020, 1:44 p.m. EDT An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Solving for laminar flow using Comsol - YouTube k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) For example, if ramping P over values of: 0.2,0.4,0.6,0.8,1.0 the nonlinear solver may fail to converge for a value of 0.8. The Automatic predictor setting will use the constant predictor when a segregated solution approach is being used, and use the linear predictor when the fully coupled approach is used. When you use an iterative solver, COMSOL Multiphysics estimates the error of the solution while solving. Tutti i diritti sono riservati. Feature: Stationary Solver 1 (sol1/s1) Adaptation gives the same mesh: adaptmesh1. Learn how your comment data is processed. Therefore, it is recommended to use Adaptive Mesh Refinement which will automatically refine the mesh only in regions where it is needed, and coarsen the mesh elsewhere. Different physics have different default solvers. If it is not clear that any of the above strategies are working, it is useful to take a more general approach to verifying the general validity of the model. If a good estimate to the solution field is known, this can be entered as an an expression in the Initial Value field. The default solver for most 3D models is an iterative solver, which is more sensitive to ill-conditioned problems. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. With the exception of some thermal problems however, it is often difficult to estimate the solution, so alternative approaches are needed. A nonlinearity can be introduced into the model either in the governing equation, or by making any of the material properties, loads, or boundary conditions dependent upon the solution. Function: / Failed to evaluate expression. The segregated approach, on the other hand, solves sets of unknowns separately. "After the incident", I started to be more careful not to trip over things. Most multiphysics problems are nonlinear. listed if standards is not an option). I use comsol multiphysics 5.2a and . When the difference in the computed solutions between successive iterations is sufficiently small, or when the residual is sufficiently small, the problem is considered converged to within the specified tolerance. replace it with the expression: What is \newluafunction? For more details, see: Performing a Mesh Refinement Study, Mesh refinement may often need to be combined with load or nonlinearity ramping and may require a set of studies, first starting with a relatively coarse mesh for nonlinearity ramping, refining the mesh, and the ramping further on the refined mesh. Extending this logic, if one wants to solve for any arbitrary load on a nonlinear system, it makes sense to solve a sequence of intermediate problems with gradually increasing load values and using the solutions from each previous step as the initial condition for the next step. The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. Such a large difference in the materials properties can be challenging. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. The advantages of the continuation method are two-fold. You can write the discrete form of the equations as f(U) = 0, where f(U) is the residual vector and U is the solution vector. - Feature: Stationary Solver 1 (sol1/s1) So far, weve learned how to mesh and solve linear and nonlinear single-physics finite element problems, but have not yet considered what happens when there are multiple different interdependent physics being solved within the same domain. Perhaps this approach could be adapted to represent your model. Knowledgebase 1260: What to do when a linear stationary model is not solving, Knowledge Base 1240: Manually Setting the Scaling of Variables, What to do when a linear stationary model is not solving, Knowledge Base 1254: Controlling the Time Dependent solver timesteps, 2023 by COMSOL. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. The idea behind the GCRO-DR method is to retain the subspace determined while solving previous systems and use it to reduce the cost of solving the next system. Building on these topics, we will now address how to prepare your mesh for efficiently solving nonlinear finite element problems. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. Sometimes, reducing the model complexity can be quite challenging and it can be better to start from as simple a case as possible and gradually increase the complexity. However, load ramping will not work in all cases, or may be inefficient. The other low-level default settings within the Stationary Solver are chosen for robustness. Any trademarks referenced in this document are the property of their respective owners. As we saw previously in the blog entry on Solving Nonlinear Static Finite Element Problems, not all nonlinear problems will be solvable via the damped Newton-Raphson method. Within either of these features, it can also be helpful to enable the Results While Solving option, as shown in the screenshot below, to visualize the iterations being taken during the solution. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. comsol - using stationary solution as the initial conditions in time This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. rev2023.3.3.43278. Few days back i was also facing this problem in . Can I tell police to wait and call a lawyer when served with a search warrant? COMSOL makes every reasonable effort to verify the information you view on this page. Leverage your professional network, and get hired. The "Values for dependent values" in study step settings should be set to the default ("Physics-controlled" in 5.2). P&S Comsol Team: Manuel Kohli, Raphael Schwanninger, Feature: Stationary Solver 1 (sol1/s1) Therefore, an initial value of zero is almost always reasonable if a very small load is applied. Staging Ground Beta 1 Recap, and Reviewers needed for Beta 2, Simulation of effect of heated resistance on temperature distribution in laminar flow, COMSOL: Diffusion in Transport of Diluted Species Produces Unphysical Results. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. Posted 26 set 2019, 11:57 GMT-4 For example, in Solid Mechanics, if the Poisson Ratio set to 0.5, then the model will not solve, as this value in incompatible with the theory of linear elasticity. This is a review for cards & stationery in Brea, CA: "Love this store!!! Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. My comment is perhaps a bit nave but it seems to me that you could simply deactivate the term of the background field equation but keep its connexion to the solid to get what you want. Nonlinearities arise as a consequence of the governing equation, as a material nonlinear expression, or as a coupling term between physics. Your internet explorer is in compatibility mode and may not be displaying the website correctly. With respect to multiphysics couplings, rather than solving the problem using a fully coupled approach (the default) solve the problem sequentially, with one physics being solved after another. There are also cases when an extremely poor quality mesh leads to an ill-conditioned problem, This issue often arises in combination with, and as a consequence of, geometries that have extreme aspect ratios. That is: It is also possible to compute the derivative of the solution with respect to the continuation parameter and use that derivative (evaluated at the iteration) to compute a new initial value: where is the stepsize of the continuation parameter. COMSOL makes every reasonable effort to verify the information you view on this page. For example, in a Solid Mechanics (wherein the software is solving for the displacement field within the solid) applying two opposite and equal Boundary Load conditions on a part is not sufficient to define the displacement. Stationary (time-invariant) models with nonlinearities may converge very slowly. Dun & Bradstreet gathers Office Supplies, Stationery, and Gift Retailers business information from trusted . Segregated approach and Direct vs. Here we introduce a more robust approach to solving nonlinear problems. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. The issue here has do with the iterative algorithm used to solve nonlinear stationary models. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Mesh There will always already be either a Segregated or Fully Coupled feature beneath this. If you do not hold an on-subscription license, you may find an answer in another Discussion or in the Knowledge Base. 0 Replies, Please login with a confirmed email address before reporting spam. The software then computes an initial solution and from there it iteratively re-computes the solution, taking into account how these intermediate solutions affect the nonlinearities. Use a manually defined mesh to avoid elements with extreme aspect ratios and perform a mesh refinement study, as described here: Performing a Mesh Refinement Study, For problems that are ill-conditioned, using a direct solver is often called for. Review the model setup to resolve these. The latter method is known as the Continuation Method with a Linear predictor, and is controlled within the Study Configurations as shown in the screenshot below. That is, they are tuned to achieve convergence in as many cases as possible. This information is relevant both for understanding the inner workings of the solver and for understanding how memory requirements grow with problem size. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version $140,000.00, $120,000.00 Set "Values for dependent variables" in study step settings to User ControlledSolutionYour Stationary Study. Use this parameter to modify the nonlinearity expressions in the model. Acidity of alcohols and basicity of amines. Why do many companies reject expired SSL certificates as bugs in bug bounties? That is: It is also possible to compute the derivative of the solution with respect to the continuation parameter and use that derivative (evaluated at the iteration) to compute a new initial value: where is the stepsize of the continuation parameter. Knowledgebase 1260: What to do when a linear stationary model is not solving, Knowledge Base 1240: Manually Setting the Scaling of Variables, What to do when a linear stationary model is not solving, Knowledge Base 1254: Controlling the Time Dependent solver timesteps. Top 10 Best Stationary Stores in Brea, CA - December 2022 - Yelp Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Wrong ordering of study steps. Repeat this for every nonlinearity of the model. COMSOL Blog Series on COMSOL Multiphysics Solvers Multiphysics problems are often nonlinear. Linear solvers. For example, in an Electric Currents problem, you may want to consider a system of materials including a good conductor such as copper (with an electric conductivity of ~6e7 S/m) and an insulative material such as glass (which can have electric conductivity of ~1e-14 S/m.) That is: Even if the forces on a part are opposite and equal, this is not sufficient information to say where the part is, so you must add some other condition, such as as Fixed Constraint to constrain displacement. That is, they are tuned to achieve convergence in as many cases as possible. Knowledgebase 1260: What to do when a linear stationary model is not solving, Knowledge Base 1240: Manually Setting the Scaling of Variables, What to do when a linear stationary model is not solving, Knowledge Base 1254: Controlling the Time Dependent solver timesteps, Galleria dei Modelli e delle App di Simulazione, 2023 da COMSOL. Improving Convergence of Nonlinear Stationary Models - COMSOL Use this parameter to modify the nonlinearity expressions in the model. See if these can be used instead of explicitly modeling parts with high-aspect ratio geometries. Why is there a voltage on my HDMI and coaxial cables? With sufficient simplification, a model can be reduced to a linear problem, and if this simplified model does not converge, see: What to do when a linear stationary model is not solving. listed if standards is not an option). That is, they are tuned to achieve convergence in as many cases as possible. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version That is: It is also possible to compute the derivative of the solution with respect to the continuation parameter and use that derivative (evaluated at the iteration) to compute a new initial value: where is the stepsize of the continuation parameter. Not entering required material parameters. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. The coupling terms between the different groups are thus neglected. Once the error estimate is small enough, as determined by the convergence criterion (20-16) the software terminates the computations and returns a solution. Using the first order optimality. Posted 26 set 2019, 17:57 CEST Mesh Version 5.3 0 Replies. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Such problems must solved in the time domain. What is the purpose of non-series Shimano components? Multiscale Modeling in High-Frequency Electromagnetics. listed if standards is not an option). In this blog post we introduce the two classes of algorithms that are used in COMSOL to solve systems of linear equations that arise when solving any finite element problem. k(T) = 10[W/m/K]*exp(-(T-293[K])/100[K]) However, it is usually not possible to know this ahead of time. If all of the above approaches have been tried and you are certain that the problem itself is well-posed, consider that the nonlinear problem may not, in fact, have a stationary (time-invariant) solution. Under Initial values of variables solved for, the default value of the Settingslist is Physics controlled. (Frequency Domain should be the last step) You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Wrong ordering of study steps. This guide applies solely to nonlinear stationary models. If these settings have been manually changed, you will see a small star symbol on the Solution feature, as shown in the screenshot below. At a value of P=0 the above expression is linear, and at a value of P=1 the expression is equal to the original nonlinear expression. If it is not clear that any of the above strategies are working, it is useful to take a more general approach to verifying the general validity of the model. That is, start by first solving a model with a small, but non-zero, load. The memory requirements will always be lower than with the fully coupled approach, and the overall solution time can often be lower as well. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. COMSOL makes every reasonable effort to verify the information you view on this page. It is sometimes necessary to manually scale the dependent variables. Understanding stationary and time dependent solver COMSOL Multiphysics In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. GCRO-DR is a method for Krylov subspace recycling. This involves a systematic reduction in the model complexity. For more details, see: Performing a Mesh Refinement Study, Mesh refinement may often need to be combined with load or nonlinearity ramping and may require a set of studies, first starting with a relatively coarse mesh for nonlinearity ramping, refining the mesh, and the ramping further on the refined mesh. (Frequency Domain should be the last step) I am solving a linear stationary finite element model but the software is not solving. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. Again, introduce a Global Parameter that gets ramped from exactly zero to one. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. At low flow speeds the flow solution will be time invariant, but at higher flow rates there will be vortex shedding, a time-varying change in the flow field behind the cylinder. This case is generally difficult, or impossible, to solve since this material property is non-smooth. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. In this page, we will be sharing some common errors that might occur in Comsol and their solutions. This case is generally difficult, or impossible, to solve since this material property is non-smooth. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. There will always already be either a Segregated or Fully Coupled feature beneath this. This information is presented in the context of a very simple 1D finite element problem, and builds upon our previous entry on Solving Linear Static Finite Element Models. Assuming a well-posed problem, the solver may converge slowly (or not at all) if the initial values are poor, if the nonlinear solver is not able to approach the solution via repeated iterations, or if the mesh is not fine enough to resolve the spatial variations in the solution. What did people search for similar to stationary stores in Brea, CA? Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere. Making statements based on opinion; back them up with references or personal experience. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. (COMSOL Multiphysics) : In such cases, see if one material (or the other) can be omitted from the analysis completely. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields.